Process for reducing oxide defects in investment castings

ABSTRACT

In casting alloys that are prone to the formation of dross such as single or complex oxides of one or more of the metals making up the alloy or crucible, such dross is filtered out by a ceramic filter positioned in the filler for the mold. More specifically, oxides formed in the melt are removed by a filter made up of ceramics formed from metal oxides since the oxides have a tendency to adhere to each other.

United States Paten [191 Curran et al.

[ PROCESS FOR REDUCING OXIDE DEFECTS IN INVESTMENT CAST-INGS [75] Inventors: Patrick M. Curran; Geoffrey R.

Brazer, both of Manchester; John S. Erickson, Colchester, all of Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: Sept. 21, 1972 [21] Appl. No.: 291,126

[52] US. Cl 164/134, 164/60, 164/358, 210/24 [51] Int. Cl. B22c 9/08 [58] Field of Search 164/60, 134, 358; 210/24, 210/69 [56] References Cited UNITED STATES PATENTS I 2,835,007 5/1958 Hoefer .L 164/134X in] 3,815,661 June 11, 1974 3,405,220 10/1968 Barrow et a1. 13/27 3,483,913 12/1969 Grosvenor et al. l 164/134 X 3,737,303 6/1973 Blayden et al. 210/69 X Primary Examiner.l. Spencer Overholser Assistant E.\'aminer--John E. Roethel Attorney, Agent, or Firm-Charles A. Warren [5 7] ABSTRACT In casting alloys that are prone to the formation of dross such as single or complex oxides of one or more of the metals making up the alloy or crucible, such dross isfiltered out by a ceramic filter positioned in the filler for the mold. More specifically, oxides formed in the melt are removed by a filter made up of ceramics formed from metal oxides since the oxides have a tendency to adhere to each other.

8 Claims, 1 Drawing Figure PROCESS FOR REDUCING OXIDE DEFECTS IN INVESTMENT CASTINGS BACKGROUND OF THE INVENTION The presence of nonmetallic inclusions or dross in the alloy has been a major source of casting rejection is forming such cast parts as turbine blades and vanes since such dross is a defect in the casting. In some parts the dross is on the surface of the cast article but in many cases the dross is located internal to the casting. Such dross is generally an oxide of one or more of the metals of the alloy or crucible and the formation of these oxides may occur due to furnace conditions or they may be from oxidation caused during the making of the master melt.

SUMMARY OF THE INVENTION One feature of the invention is an arrangement for removal of the dross during the pouring of the metal alloy into the mold. Another feature is the use of a filter through which the molten alloy is poured and which will remove dross from the molten metal during the casting of either hardware or master melt.

According to the invention filter elements made up of a ceramic such as aluminum oxide are positioned in the filler cup for the mold and collect on the surface the oxides in the alloy being poured. The energy relations between the interfaces of the ceramic'elements and the oxides in the alloy favor adherence of the dross to the filter elements. The invention has broad application to the removal of many oxides from the molten alloys; the invention has particular utility in the removal of hafnium dioxide from alloys in which hafnium is an alloy ing element and in this event the ceramic may be in the form of spheres and may be formed from aluminum oxide.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a sectional view through a mold showing the invention.

The device is shown in use in a mold for making a directionally solidified casting which in this arrangement is a columnar grained cast article. Referring to the drawing, a shell mold 2 is formed by shell molding techniques for use in the lost wax method of casting and such molds are normally used in a vacuum or in an inert atmosphere. The mold rests on a water-cooled chill block 4 of conventional construction. The mold shown has two article forming portions 6 which are duplicates in the arrangement shown. At the base of the mold is a growth zone 8. Directly above the growth zone is the article forming portion 6. At the top of the article forming portion is a header 12 which communicates with the several mold sections and which also carries a filler cup 14. It will be understood that this type of mold is of a well known construction as described in Barrow, et al. US. Pat. No. 3,405,220. The invention is equally applicable to molds adapted for forming single crystal articles as described in the patent to Piearcey US. Pat. No. 3,494,709 and it has use in more conventional molds of the type used for casting articles in which the grain is random or equiaxed.

In accordance with the invention, the filler cup has a perforated baffle 16 therein and on this baffle are positioned a large number of ceramic elements 18 preferably in the form of spheres. These elements or spheres are preferably made of a metal oxide such as aluminum oxide (A1 0 Other oxides may be equally usable such as BeO, SiO MgO, ZrO MgAl Q, Mg SiO ZrSiO The invention is presently in use in the removal of hafnium dioxide (HfO from investment cast directionally-solidified gas turbine blades and vanes and the filter used in this case is ceramic balls formed from aluminum oxide and the balls are preferably about 1/2 inch in diameter. Several layers of these balls are desirable so that the molten alloy in flowing over the balls is caused to move in a circuitous path for exposure of more molten alloy to the surface of the balls.

The interfacial energy between the hafnium dioxide dross and the surface of the balls is sufficiently low to cause the dross to adhere to the surface of the balls and thereby be removed from the alloy before it flows from the filler cup into the article portions of the mold. Prior to the time that the filter was in use, the dross suspended in the liquid alloy migrated to the surface of the mold and adhered to it. The result was the accumulation of the oxide on the surface of the cast article where it could readily be seen and where it was detrimental and would result in the discard of that particular cast article.

When the alloy used is cast in the more conventional molds so that the grain structure is equiaxed, the hafnium dioxide dross is more frequently internal to the casting and the dross particles tend to be rounded in shape. The internal location of these defects is believed to be related to the rapid solidification that occurs in conventional equiaxed castings which trap the dross within the casting before it can migrate to the mold surface.

This filter has been used particularly with the alloy nwnas WA 1. .2. which s ,thqf vwin p tion:

Min. Max.

Carbon 0.08 0. l 4 Manganese 0.20 Chromium 8.00 10.00 Cobalt 9.00 l [.00 Tungsten l 1.50 l3.50 Columbium 0.75 1.25 Titanium 1.75 2.25 Aluminum 4.75 5.25 Hafnium l.75 2.50 Boron 0.010 0.020 Zirconium 0.20 Nickel Remainder Where the balls used in the filter were about 1/2 inch diameter balls made of aluminum oxide positionedin the filler cup to a depth of about 2 to 3 inches, substantially all of the'hafnium dioxide was removedbyadhering to the balls making up the filter.

When this alloy was cast without the use of the filter, the hafnium dioxide dross was found to be spread out along the alloy-mold interface and appears somewhat agglomerated below the surface of the casting. It is believed that since the directional-solidification cycle in producing directionally solidified parts kept the alloy liquid in the mold for an extended period of time, the dross had an opportunity to migrate to the shell surface and adhere to it. when the filter was used, substantially all of these imperfections disappeared and the hafnium dioxide was found to have accumulated on the spheres making up the filter.

Another alloy for which the filter system has been u e is t e alloy WA 1.455 ch h s q lqwine composition:

Min. Max.

Carbon 0.08 0.13 Manganese 0.20 Chromium 7.50 8.50 Cobalt 9.50 10.50 Molybdenum 5.75' 6.25 Aluminum 5 .75 6.25 Tantalum 4.00 4.50 Hafnium l l l .60 Titanium 0.80 l .20 Boron 0.0 l 0 0.020 Nickel Remainder This alloy was cast in the form of a conventional equiaxed casting. Without the filter, the hafnium dioxide dross appeared internal to the casting and the individual particles tended to be rounded in shape. The fact that the dross was located internally in the casting is believed to be a result of the rapid solidification which occurs so that the dross is trapped before it can migrate to the mold surface. In pouring this alloy with a filter in position, it was found that substantially all of the hafnium dioxide dross was removed by the filter before it had a chance to reach the article forming portions of the mold. In using the filter in this way, the filter itself is not heated to as high a temperature as it would in the production of directionally solidified hardware but in spite of the fact that the filter temperature is considerably colder, there was no problem in an adequate flow of molten alloy through the filter and sound castings were achieved.

Another conventionally cast superalloy used was PWA 655 which has the following composition:

Carbon max 0.20 Chromium l3.00 l5.00 Molybdenum 3.50 5.50 Columbium 1.00 3.00 Tantalum Titanium 0.75 1.25 Aluminum 5.50 6.50 Boron 0.005 0.0 l Zirconium 0.05 0.l2 Cobalt max 1.00 Nickel Remainder This alloy when cast conventionally is prone to dross formation but with the use of the filter substantially dross free, sound castings with an acceptable grain size were achieved even though the part produced was 12 inches in length. The dross formed in this alloy is found were used. The filter concept has been used in the production of experimental master melts where cleanliness of the master melt is essential.

It is also expected. that other oxides than aluminum oxide would be efiective without serious modification of the necessary surface energy relations. Such filter materials could be, for example, BeO, SiO MgO, 2x0, 7

7 size of the spheres to make sure that the increase in the to contain A1 0 Mg(AlCr) O and NiAl Q, and it was 7 found that these oxides were readily removed from the molten alloy stream. It is believed that the filter will also be usable in removal of other dross oxides which occur in air and vacuum melted cobalt, nickel and iron base alloys, such oxides being CoCr- O Fe O MgFe- 0 NiAl O FeSiO and CoFe O among others. Such alloys are, for example, the well known high strength, high temperature alloys, frequently referred to as superalloys used especially in the hot areas of gas turbines.

In addition to using the filter in making article castings for use, it has been found equally usable in the production of master melts such as used by the investment casting industry. These melts often contain an excessive amount of detrimental oxides and such oxides would be routinely removed by the filter if such a filter resistance to the metal flow is not such as to prevent filling of the mold within an adequate time.

We claim: '1. In the manufacture of cast articles from high strength high temperature alloys, the steps of:

forming a mold with a filling cup communicating'with the article cavities in the mold; positioning ceramic metallic oxide particles in said cup; pouring aihigh strength high temperature molten alloy which includes a readily oxidizable element into said cup to fill said article cavities; and

causing a removal of dross including oxides of said element from the molten alloy as the alloy flows over the ceramic particles by adherence of the oxides of the element to the particles.

2. The process of claim 1 in which the ceramic particles are aluminum oxide spheres and the molten alloy includes hafnium.

3. The process of claim 2 in which some of the hafnium forms an oxide and the aluminum oxide particles remove the hafnium by reason of the adherenceof the hafnium oxide to the aluminum oxide.

4. In the manufacture of directionally solidified cast articles, the steps of:

forming the mold having at least one article cavity, a

growth zone at the bottom thereof and a filling cup at the top;

positioning the mold on a chill plate;

placing'in the filling cup a filter consisting of a ceramic made of an oxide of metal;

heating the mold with the oxide in the filler cup to a temperature above the melting point of the high strength high temperature alloy to be cast, the latter including an element that oxidizes readily during the melting of the alloy,

pouring the melted alloy into the cup to fill the article cavity in the growth zone; and

causing the removal of the dross from the molten alloy as the metal flows over the ceramic oxide particles by the adherence of the oxides of the element to the ceramic oxide particles.

5. The process of claim 4 in which the ceramic is in the'form of spheres of about l/2 inch in diameter and from an oxide of one of the metals in the alloy.

6. The process of claim 4 in which the alloy includes hafnium part of which oxidizes to hafnium dioxide and the ceramic is alumina.

7. In the casting of high strength high temperature al-.

loys of the type having a nickel or cobalt base and also 5 including as an essential element a metal subject to oxidation during the preparation of the molten alloy, the steps of:

positioning a bed of small diameter ceramic metallic oxide spheres in the filling cup for the mold;

forms hafnium dioxide and the spheres are alumina. 

2. The process of claim 1 in which the ceramic particles are aluminum oxide spheres and the molten alloy includes hafnium.
 3. The process of claim 2 in which some of the hafnium forms an oxide and the aluminum oxide particles remove the hafnium by reason of the adherence of the hafnium oxide to the aluminum oxide.
 4. In the manufacture of directionally solidified cast articles, the steps of: forming the mold having at least one article cavity, a growth zone at the bottom thereof and a filling cup at the top; positioning the mold on a chill plate; placing in the filling cup a filter consisting of a ceramic made of an oxide of metal; heating the mold with the oxide in the filler cup to a temperature above the melting point of the high strength high temperature alloy to be cast, the latter including an element that oxidizes readily during the melting of the alloy, pouring the melted alloy into the cup to fill the article cavity in the growth zone; and causing the removal of the dross from the molten alloy as the metal flows over the ceramic oxide particles by the adherence of the oxides of the element to the ceramic oxide particles.
 5. The process of claim 4 in which the ceramic is in the form of spheres of about 1/2 inch in diameter and from an oxide of one of the metals in the alloy.
 6. The process of claim 4 in which the alloy includes hafnium part of which oxidizes to hafnium dioxide and the ceramic is alumina.
 7. In the casting of high strength high temperature alloys of the type having a nickel or cobalt base and also including as an essential element a metal subject to oxidation during the preparation of the molten alloy, the steps of: positioning a bed of small diameter ceramic metallic oxide spheres in the filling cup for the mold; pouring the molten alloy over this bed of spheres in filling the mold; and causing adherence of the oxides of the metal to the ceramic spheres as the molten alloy flows over said spheres.
 8. The process of claim 7 in which the alloy being cast includes as an essential element hafnium which forms hafnium dioxide and the spheres are alumina. 